1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly to the improvement of an image forming apparatus for applying an electric field to an aperture to allow charged toner particles to fly or pass through the aperture thereby forming an image on an image support member.
2. Description of Related Art
There has been conventionally known one type of image forming apparatus having an aperture electrode member. FIG. 9 shows a conventional aperture electrode member 50 of this type.
As shown in FIG. 9, the aperture electrode member 50 includes an insulating layer 52, a reference electrode 53 which is continuously formed on one surface of the insulating layer 52, and control electrodes 54 which are formed on the other surface of the insulating layer 52 so as to be electrically insulated from one another. That is, the aperture electrode member 50 is so designed that the insulating layer 52 is sandwiched between the reference electrode 53 and the control electrodes 54. In addition, at least one array of apertures 55 are formed at positions corresponding to the respective insulated control electrodes 54 so as to penetrate through the three layers (reference electrode, insulator and control electrode). This array of apertures 55 (hereinafter referred to as "aperture array") is formed over the whole width of the image support member (copy sheet, not shown). This type of aperture electrode member is disclosed in U.S. Pat. No. 5,099,271.
A voltage is selectively applied between the control electrodes 54 and the reference electrode 53, using the aperture electrode member 50 having the above construction, to control the passage of toner particles (not shown) through the apertures 55. The image support member and the aperture electrode member 50 are moved relative to one another to position the image support member in a toner-particle flowing passageway and a desired image is formed on the image support member.
In the image forming apparatus using the aperture electrode member 50, as described above, formation of an image on the image support member having gradation has been performed using conversion means, for converting image data containing gradation data input from an external source into binary image-forming data. Such a printer could most probably use a dither method, which is an areal gradation method (shown in FIGS. 10A and 10B) for printing such as is used with dot matrix and laser printers, using voltage applying means for selectively applying a voltage to each of the control electrodes on the basis of the image data.
Means for determining the density of the areal gradation as described above will be described with reference to FIGS. 10A and 10B.
As shown in FIG. 10A, for example, in order to form a capital letter C as an image, plural picture elements are arranged in a C-shaped form, and each of the picture elements is provided with density. Each picture element comprises plural dots, for example, 4.times.4 dots each corresponding to each aperture. The gradation (light and shade) of each picture element is represented (adjusted) in accordance with the number of dots to be smeared (blackened) for each picture element. That is, in FIG. 10B, the density of the picture element is gradually heightened toward the right-hand picture element.
However, a large number of dots must be used to perform gradation representation in the conventional areal gradation method. In order to obtain 64-step gradation for example, an 8.times.8 dot matrix is required. An image forming apparatus having 16 dots/mm resolution provides an output image of 2 picture elements/mm when 8.times.8 dot matrix is used, and thus there occurs a problem that the resolution is greatly degraded.